The present invention relates to a control device of a spark-ignition gasoline engine.
As an art of achieving both improvements in an exhaust emission and thermal efficiency, as disclosed in, for example, JP2009-091994A, an HCCI engine where a lean air-fuel mixture is compression-ignited has been known. However, as an engine load increases, a compression-ignition combustion turns into a combustion caused by a pre-ignition with a rapid pressure increase. Thereby, combustion noise increases and abnormal combustion, such as knocking, is caused, and further, an increase in Raw NOx due to a high combustion temperature is caused. Thus, with such an HCCI engine, the compression-ignition combustion is limited to be performed only within an operating range of the engine where the engine load is relatively low, and within an operating range of the engine where the engine load is relatively high, a spark-ignition combustion is generally performed by operating an ignition plug.
Meanwhile, the engine is operated at a leaner than theoretical air-fuel ratio to generate the compression-ignition combustion, whereas the engine is operated at the theoretical air-fuel ratio to perform the spark-ignition combustion. Therefore, in the case of the spark-ignition combustion, because an amount of fresh air is determined with respect to a fuel amount according to the engine load, an intake air charging amount needs to be reduced by, for example, a throttle control by a throttle valve. However, such a throttle control increases pumping loss, therefore the number of times the throttle control is performed is desired to be as small as possible. Whereas, introducing an external EGR gas into a cylinder can reduce the amount of fresh air without reducing the intake air charging amount. Therefore, adding the external EGR control in the case of the spark-ignition combustion is advantageous in reducing the pumping loss. Further, the introduction of the external EGR is advantageous in reducing cooling loss as well as beneficial in avoiding the abnormal combustion, such as the pre-ignition and knocking, and suppressing the generation of Raw NOx.
However, in view of stabilizing the combustion, the amount of the external EGR gas that can be introduced into the cylinder is controlled to be comparatively small. Thereby, there has been a problem that although, when the engine load is high and the fuel amount and the amount of fresh air are comparatively large, the intake air charging amount is not needed to be reduced due to the introduction of the external EGR gas, when the engine load is around the middle engine load and the fuel amount is comparatively small, such as near a boundary between an engine load range where the compression-ignition combustion is performed and an engine load range where the spark-ignition combustion is performed, even if a maximum amount of the external EGR gas is introduced, the amount of fresh air needs to be reduced by the throttle control and the pumping loss is hardly reduced.
The present invention is made in view of the above situations and narrows as much as possible, in a spark-ignition gasoline engine for switching mode between a compression-ignition mode in which a compression-ignition combustion is performed and a spark-ignition mode in which a spark-ignition combustion is performed, the engine load range where an intake air charging amount is needed to be reduced, by improving combustion stability in the spark-ignition mode.
It has been found that a characteristic fuel injection mode such that a fuel is injected into a cylinder with a comparatively high fuel pressure at a timing near a compression top dead center shortens the combustion period and, thereby, combustion stability improves. Thereby, this fuel injection mode enables a large amount of EGR gas to be introduced into the cylinder and, as a result, leads to the completion of the present invention.